K8MM-V unmountable boot volume

Question

motherboard  k8mm-v
ram 512        pc400
psu              300 watts
video            msi ge force fx5500 agp 8x
hdd              hitachi desk star 80 gb ide 80pin ribbon
processsor     athlon xp 64 3000+
os                win xp professional  sp2
drivers video  nvidia forceware release 80 version 81.98
latest bio flash and drivers form msi live update
mobo drivers via hyperion pro ver 504a
Hello fellows . The problem i have been experiencing is that the computer shows a Blue Screen after windows start booting (Like 2 seconds after the windows logo appears while we wait for windows to boot) and it inmediately resets without letting us see the error it shows after that, the MSI V CLASS First screen appears as if we were turning on the computer , and it starts booting again, repeating the process, until we boot  windows in safe mode  and sometimes it even boots 1 time normally after it has been reseted while being in safe mode. This problem started after we did a clean install and installed windows xp sp2. Before we had windows professional without the service pack and we didnt had this problem the computer worked normally. 
i reinstalled xpsp2 with another cd and the computer worked normally on the first boot. without installing any software or drivers i rebooted and  the blue screen appeared again now showing the details of the error, it says unmountable_boot_volume.
then i got in to safe mode and  restarted the computer and it  booted normally again , but if we restart after the computer boots normaly the blue screen shows again.
i noticed a process called "slow down cpu"  on the msconfig window, i unchecked the process and now the problem seemed to be ok, but still sometimes when i restart the system after the computer is used  the blue screen shows again, to be able t boot the os i have to get into safe mode and reboot.
i have to admit that after i removed the slow down cpu process the computer is behaving far more better, but i would like to get rid of the problem for good. Any suggestions or anybody had experienced this problem before?. thank you in advance.
Note that i have installed all the drivers listed on the top of this post and none had got rid of the problem.          

Answer 1

That's interesting Tiresmoke, i will sugest a psu upgrade, but let me tell you what i have done yesterday.
* First i did the memtest86 as you sugested and all the test were performed succesfully.
* Then i installed the hitachi hdd in to another computer and did a clean winxpsp2 with games and everything , we can asume that the disk is ok cause no blue screens were seen.
*After that i installed a smalled hdd 40 pin 8 gb on to the blue screen computer (athon 3000) , i managed to make a clean winxpsp2 and tested games with no problem!!! no blue screaaaaamssss!!!!.
You can imagine the state of confusion i had so far.  i had to conclude that either the IDE0 on the mobo was having problems, or the ribbon had problems.
I didint had another 80 connector ribbon i proceded to change the ribbon from ide0 to ide1.  let me tell you that so far my friend havent had any bluescreens but he is still testing.  Im thinking of changing the mobo at the shop.
Please tell me what do you think, ill keep you posted. Thanks pal